COMBINED TECHNOLOGY OF REPAIRING AND
HARDENING OF MACHINE PARTS
Alexey Kossenko1, Yury Kuznetsov 2, Michael Zinigrad1
1
Ariel University Center of Samaria, Ariel, Israel
2
Orel State Agricultural University, Orel, Russia
Abstract
The combined technology of wear proof coatings by means of
Cold Gas Dynamic Spraying (CGDS) followed by treatment of the
sprayed layer by Micro Arc Oxidation (MAO) has been developed. This
technology provides good physical and mechanical properties and high
performance of coatings obtained. Theoretical assessment of powder
particles – substrate interaction at gas-dynamic spraying has been made.
Test data relating to the properties of the coatings obtained, and general
technological recommendations on practical application of the proposed
technology are provided.
1. Introduction
It is known that higher wear resistance of most machine parts is
achieved by using coatings with required parameters. Wear of machine
and equipment components is known to be 0.1 – 3.0 mm for different
types of materials. Worn surfaces are industrially repaired by various
methods, such as using oversize spare parts; welding and surfacing;
spraying; plastic straining, various electrochemical processes, and other
types of hard surfacing [1,2]. Unfortunately, these feature a lot of major
disadvantages:
- they fail to always ensure the required endurance of friction
pairs;
- certain materials (and technologies) used are limited by strict
sanitary standards, which particularly concerns surfaces coming in
contact with food environment and food products;
- high complexity and high cost of the technological equipment
used;
- technologies are inconsistent with ecological standards and
require costly special-purpose protection.
The most promising of the current repairing technologies are
certain types of thermal spraying, and fast developing micro arc
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oxidation. Studies of the special nature of thermal spraying and micro
arc oxidation for various types of materials, including steels and valve
metals have revealed a number of benefits and flaws.
Thermal spray technology does not necessarily ensure efficient
coatings, which would be highly resistant to thermal cycling due to their
high friability and poor adhesion. Furthermore, high porosity and
roughness of coating surface require mechanical treatment of articles
aimed to preserve their sizes and ensure adequate quality of surface.
Cold gas dynamic spraying, which is a relatively new method of coating,
has been widely used since recently [3, 4]. With CGDS, powder
particles (a plastic metal powder or composite powder with less than 50
µm particles) are accelerating up to 300-1,200 m/s by a heated Laval
nozzle-generated ultrasound high-pressure gas jet. This creates a solid
high-adhesive (35-40 MPa) coating of low strength metals by using
relatively simple equipment [5, 6].
As regards micro arc oxidation [7, 8], it enables high-adhesive
oxide-ceramic layers with adequate wear- and heat resistance properties
depending on porosity and roughness of the treated material in order to
maintain the initial dimensions and geometric parameters of detail and,
possibly, to rule out subsequent mechanical treatment of the coated
details. However, МАО is used in valve metals alone [9], and cannot be
applied in treatment of steels used to produce most of the machine and
equipment parts.
This paper deals with the results of consequent application of both
of these methods (CGDS and МАО) and the development of combined
repairing and hardening technology to integrate their advantages. The
resulting technology is recommended for industrial use, including
repairing of details made of aluminum alloys, alloys steels, carbon steels
and corrosion-resistant steels. Expected increase in life cycle of
reconditioned and hardened articles is about 150-200% versus brand
new parts.
2. Theoretical basis
Assessment of powder particles–substrate interaction at gasdynamic spraying
With all its advantages, ultrasound CGDS is largely different from
conventional gas-thermal spraying [10]. According to [11], CGDS is
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performed within a specific speed range vp1  v p  vp2 , with critical
speeds vp1 and vp2 highly dependant on particle size, pre-impact particle
and substrate temperature (spraying temperature T0), and particle [12]
and substrate material.
At vp < vp1, particles rebound from the substrate is attributed to an
insufficient number of links formed at particle-substrate impact, with the
adequate number of links not encouraged by a high strain heating.
Vp1 – is a minimal critical speed of particle when knocked against
the surface, as described by the following equation:
5/7
 126k TD E1 
vp1  
2 2 
 3  a 
1/7
 D4 


  
 5Ea 
 exp  

 7k T0 
where Е1 – is an energy of single coupling of two atoms;
activation energy;
k
– is Boltzmann constant;

(1)
Еа
– is an
– is Planck constant;
Т D – is Debye temperature ( Т D = 428 К); а – is crystal lattice period;

– is reconditioning factor, which is speed relationship before and
after impact against the substrate;  – is density of sprayed particles
material; Т 0 – is initial particle and substrate temperature; D – is a nondimensional mechanical properties-depending parameter:
D
3  1   2 1   2 


,
4  E
E  
(2)
where  and  – are Poisson ratios of contacting bodies; E and E – are
Young's modulus of contacting bodies.
This critical speed is non-dependent on sprayed particles
diameter, being mainly a function of mechanical properties of particle
and substrate material. Critical speed of aluminum substrate-sprayed
aluminum particles (TD=428 K,
а =410–10
vp1384 m/sec.
80
m) at
Т 0 =300K
will be
If vp>vp2, particle is detached from the substrate, though the
maximum possible number of links is formed during contact. It is
attributed to a high reserve of residual particle energy and substrate
elastic compression energy at the final stage of plastic strain.
The following equation has been obtained to determine critical
speed vp2:
5/6
 44E  
vp2   2 21 
 a 
D1/3
,
  d 5/6
(3)
where d – is sprayed particles diameter, with the remaining variables
similar to those used in equation (1).
Critical speed vp2 is therefore not only dependant on mechanical
properties of particle and substrate material, but also on sprayed particles
diameter.
Finally, it should be noted that gas-dynamic spraying is only
possible at vp1<vp2. This imposes further restrictions on the size of
particles at a given spraying temperature: d<dk( Т 0 ), where dk – is a
critical diameter of particles that corresponds to Т 0 .
By using (1) and (3), we obtain the following critical diameter of
particles:
1/7
 E  6 
4,77
d k (T0 )  3/7 6/21   2 21 6 6 
 D
  a k  TD 
 6 Ea
exp 
 7k T0

.

(4)
Equations (1) and (3), as obtained to determine critical speeds and
maximum diameter of sprayed particles (4) at gas-dynamic spraying,
enable optimization of parameters aimed to ensure an optimal coatingsubstrate adhesion.
3. Experimental procedures
3.1. Sample preparation
Coatings were generated on square coupons (50x50x4 mm) of
aluminum UNS A91200 and steel UNS G43400.
Aluminum coating has been sprayed on samples by means of lowpressure cold gas dynamic spraying system (below 0,9 MPa), Dimet-405
(Fig. 1). Powder А80-13 (Al – base, Al2O3, Zn; Dimet) have been used.
81
Compressed air was used as an actuation gas. Before spraying, the
samples have been cleaned up and activated by sand blasting using 200
mesh Al2O3 at the same equipment.
A homemade in Advanced Materials Laboratory of the Ariel
University Centre of Samaria alternating-current micro arc oxidation
system (Fig. 2) was used for MAO. The MAO device contains a 40 kW
AC power supply with a 50 Hz modulation, an electrolyte bath, a stirring
and cooling system. Current density was 15-20 A/dm2.
The electrolyte consisted of an aqueous solution of sodium silicate
(6 g/l) and potassium hydroxide (3 g/l).
Fig. 1. Cold Gas Dynamic
Spraying System Dimet 405
Fig. 2. 40kW Micro Arc Oxidation System
3.2. Characterization
Adhesion strength of the resulting coatings was tested at PosiTest
Pull-Off Adhesion Tester (DeFelsko Corporation, USA) according to
ASTM D4541-95e1 [13], ASTM C 633 – 79 [14].
Metallographic tests were conducted at ZEISS Axiolab A
equipped with an image review system and SEM JEOL 35CF.
Coating thicknesses (total thickness of coating and oxidized layer
thickness) were measured using a digital CM-8825 (Ferrous & Non
Ferrous type) coating thickness gauge, which coating thickness with an
accuracy of 0.1 µm.
Comparative coating wear-resistance tests (both for cold gasdynamic spraying and МАО) were conducted according to ASTM G 99
– 95a [15] using «rotating disk – fixed pin» pattern at constant load and
82
rotation speed. Non-coated aluminum alloys (UNS A04131, UNS
A03561 and DIN G-AlSi12) and CGDS-formed coating with no МАО
hardening were used as a reference. Tests were lasting for 200 hours.
Microhardness of the resulting coating was checked at
microhardness tester Buehler Micromet Microhardness 2103.
4. Results and discussion
4.1. Coating structure
Approx. 200 µm sprayed powder layer was exposed to a 120-150
µm deep micro arc oxidation for about 100-120 minutes. Microhardness
of the oxidized layer was 1200-1500HV. Typical microstructure of the
resulting coating after CGDS and MAO is shown in Fig. 3: the coating
contains a residual sprayed aluminum layer (50-100 µm) with aluminum
oxide inclusions, and 100-150 µm aluminum oxide layer (MAO
coating).
a
b
c
Fig. 3. Typical microstructure of hardened layer before polishing (a – Al2O3; b –
sprayed layer; с – base steel)
83
The MAO coating contains three layers: the loose layer, the
compact layer and the transition layer. The thicknesses of these layers
obtained in the experiment were 50μm, 50μm and 30μm respectively.
The loose layer is composed of many spherical particles with the
diameters of 5~10μm on the surface. There are many minute cracks
between particles, so these volcanic shape particles can be easily
polished off using abrasive paper. The hardness and corrosion resistant
abilities of the MAO coating are mainly supplied by the compact layer
because of the dense structure of this layer.
During the procedure of the MAO, the thickness of the coating
increases quickly in the initial stage. The growth speed slows down
when the MAO coating achieves certain thickness. In this time, the
MAO procedure is still working but the growth and the elimination of
the coating achieve basically balance. In certain scope, with the current
density’s increase, the thickness of the coating raises.
4.2. Adhesion Strength of Coatings
Results of adhesion studies on CGDS coatings [16] on aluminum
alloys and carbon alloy steels are shown in Figure 4.
Particles of the sprayed powdered material are speeding up, as air
pressure is rising in CGDS plant spraying chamber, and therefore an
maximum adhesion strength of coatings is achieved (Figure 4а). The
maximum possible pressure in the spray chamber depens on design
features of the plant.
As air heating temperature pressure is rising, adhesion strength of
coatings reduces (Figure 4b). This is attributed to higher activity of
sprayed particles. Therefore, not only the particles with an adequate
kinetic energy will adhere onto the sprayed surface, but also the particles
with lower kinetic energy and higher temperature.
Adhesion strength of CGDS and gas-flame sprayed coatings is
gradually rising proportionate to the increase in roughness of the sprayed
surface. The maximum adhesion is achieved at: Rz = 60-120µm.
The experimental data obtained support our theoretical
assumption that solid particle – substrate interaction associated with
CGDS is not only dependent on heating temperature and air pressure in
the spraying chamber, but also on sprayed particle size (figure 4c). There
are always particles of such size which would ensure their detachment
from the substrate, whatever their speed may be, even if the maximum
possible number of links has been created at contact.
84
Fig. 4 – Adhesion strength – CGDS mode dependence (1 – aluminum substrate;
2 – steel substrate): a) dependence on air pressure in the spraying chamber (air
heating temperature (const) – 400 0С); b) dependence on air heating temperature
in the spraying chamber (air pressure in the spraying chamber (const) – 0.7
MPa); c) dependence on powdered material fraction (air pressure in the spraying chamber– 0.7 MPa, spraying distance– 15 mm, air heating temperature –
400 оС).
85
This evaluation of coating adhesion strength shows that under the
given interaction conditions, particles with elastic energy and adhesion
energy have the same order of magnitude, with elastic energy of
compression gaining a major importance in solid spraying. Therefore,
relatively small fractions of sprayed powder shall be used to alleviate the
effect of elastic rebound of particles (≤ 60 µm).
To maintain adequate strength properties of coatings created by
CGDS and МАО hardening, transition area between the substrate and
МАО-hardened layer shall be at least 70…100 µm thick.
The following main CGDS factors affecting the adhesion
strength of coatings have been selected: air pressure (х1) and heating
temperature (х2) in the spraying chamber, and powder fraction (х3).
The following regression equation was obtained by calculations
[11]:
Y  61,85  0,118 x1  18,537 x2  0,049 x3  0,071x1 x2 
0, 452 х1 х2  0,0001х12  33,79 х22  0,002 х32
(5)
According to the equation, adhesion strength is mainly affected by
air pressure in the spraying chamber and air heating temperature.
Quantitative study of adhesion strength for MAO coatings formed
in КОН-Na2SiO3-type electrolyte showed no blistering or stripping of
coatings on control surfaces, regardless of the type of electrolyte, current
density, and oxidation time.
4.3. Abrasive Wear Tests
Results of wear resistance studies for proposed coatings are
shown in Figure 5. According to [16, 17], wear resistance of hardened
CGDS coatings is 7 – 7.8-fold higher than of non-hardened coatings, and
5 – 6-fold higher than of aluminum alloys.
86
0.8
wear, g
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
A04131
A03561
G-AlSi12
materiall disk
CGDS
CGDS+MAO
materials disk
materiall pin (steel AISI 52100)
Fig.5. Wear values for "disk-pin" samples (СGDS – gas dynamic spraying;
CGDS+MAO – gas dynamic spraying and micro arc oxidation).
Table 1 shows the wear rates of friction couples compared.
Table 1 – "Disk-pin" friction couples wear rate assessment
Disk material
Pin material
1
2
AISI 52100
steel
–
–
–
A04131
A03561
G-AlSi12 (Germany)
CGDS-formed coating
CGDS-formed
and
hardened coating
MAO-
–
Friction
couple wear
rate, g/h
3
0.0277
0.0338
0.0295
0.0397
0.0065
Analysis of the data obtained shows that wear rate of friction
couples with oxide-ceramic coated samples is 6-fold lower than of
reference friction couples with CGDS-formed coating without
hardening, and 4.1…5.2-fold lower than the wear rate of friction
couples with aluminum samples (depending on the type of alloy).
87
General view of some samples subjected to comparative wear
tests is shown in Figures 6 and 7.
а)
b)
Fig. 6 – General view of A03561 aluminum alloy sample after wear tests, 5fold magnification (а); and 500-fold magnification (b).
а)
b)
Fig. 7 – General view of hardened sprayed sample after wear tests, 5-fold
magnification (а); and 500-fold magnification (b).
5. Conclusions
This combined technology enables to form hardened high
resistant and adhesion strong layers on steel (aluminum) surfaces.
The study supports efficacy and feasibility of using combined
technology that consists of creating a powdered aluminum-containing
layer on metal surfaces (including non-valve metals) by means of CGDS
followed by sprayed layer oxidation using MAO, with required adhesion
88
strength obtained by treatment modes, and structural status of the
working surface attributed to electrolyte composition and porosity of the
interim powder sub-layer, which in its turn depends on its thickness and
spraying modes.
All performance parameters obtained are conformant to standard
requirements.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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